56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 7 1 



ignited on impact. The light would then be visible in a powerful 

 telescope. Further, the larger the aperture of the telescope, the 

 greater would be the ease of seeing the flash, from the fact that a 

 telescope enhances the brightness of point sources, and dims a faint 

 background. 



An experiment was performed to find the minimum mass of flash 

 powder that should be visible at any particular distance. In order 

 to reproduce, approximately, the conditions that would obtain at 

 the surface of the moon, the flash powder was placed in small 

 capsules, C, plate 9, figure i, held in glass tubes, T, closed by 

 rubber stoppers. The tubes were exhausted to a pressure of from 

 3 to 10 cm. of mercury, and sealed, the stoppers being painted with 

 wax, to preserve the vacuum. Two' shellacked wires, passing to the 

 powder, permitted firing of the powder by an automobile spark coil. 



It was found that Victor flash powder was slightly superior to a 

 mixture of powdered magnesium and sodium nitrate, in atomic pro- 

 portions, and much superior to a mixture of powdered magnesium, 

 and potassium chlorate, also in atomic proportions. 



In the actual test, six samples of Victor flash powder, varying in 

 weight from 0.05 gram to 0.0029 gram were placed in tubes as shown 

 in plate 9, figure i, and these tubes were fastened in blackened com- 

 partments of a box, plate 9, figure 2, and plate 10, figure i. The 

 ignition system was placed in the back of the same box, as shown in 

 plate 10, figure 2. This system comprised a spark coil, operated by 

 three triple cells of " Ever-ready " battery, placed two by two in 

 parallel. The charge was fired on closing the primary switch at the 

 left. The six-point switch at the right served to connect the tubes, 

 in order, to the high-tension side of the coil. 



The flashes were observed at a distance of 2.24 miles on a fairly 

 clear night ; and it was found that a mass of 0.0029 grams of Victor 

 flash powder was visible, and that 0.015 gram was strikingly visible, 

 all the observations being made with the unaided eye. The minimum 

 mass of flash powder visible at this distance is thus surprisingly small. 



From these experiments it is seen that if this flash powder were 

 exploded on the surface of the moon, distant 220,000 miles, and a 

 telescope of one foot aperture were used — the exit pupil being not 

 greater than the pupil of the eye (e. g., 2 mm.) — we should need a 

 mass of flash powder of 



2.67 lbs., to be just visible, and 

 13.82 lbs. or /^j.y, to be strikingly visible. 



